Monthly Archives: January 2017

Making reliable electronic assemblies is about making a good solder joint isn’t it?

So what’s the problem, plop down some paste in the right area place the component carefully down, apply heat and the solder melts. Hey presto!

Yes, if only it was that easy. The Jedec thermal profile is well known and was mentioned in a previous issue???? The problem is every part of a PCB will have a different thermal profile as will every component and joint. In the ideal world the flux needs to activate at the same time and the solder melt at the same time, but this is never going to happen.

The result of this is an electronic assembly can suffer from dry joints in places and others voids, head in pad or possibly tombstoning. It’s quite a juggling act that needs to be performed. Unfortunately it’s mainly trial an error, but knowing what to consider before going to assembly can help avoid some of these nasty niggly errors.

It’s more difficult to uniformly heat a PCB to the correct Jedec profile. Thermal shadows and hotspots in different parts of the PCB can create problems in achieving a good result. Using a longer thermal ramp up and dwell time can often help but getting heat under leadless components and BGA’s will always create difficulties.

Dry joints and voids under these components will not be immediately obvious with only x-ray inspection able to reveal any issues. Many BGA’s have hundreds of balls on them with every singe one needing to be perfectly soldered in order to operate properly and reliably.

Vapour phase reflow is becoming more popular in reflowing modern high density PCB’s as this gives a far more uniform and reliable thermal profile across the PCB being assembled, even for joints under the component such as BGA’s. It tends to be a slower way of reflowing boards, so it isn’t used as much in higher quantities, but the process gives good results across the board.

So now we know how to make a good solder joint that’s it right? The board can be put into service and will work for years – problem solved?

No.

Many PCB’s are put into service in environments that are harsh, with high levels of moisture, dust, or some kind of chemical pollution. Any one of these will affect the long term operation of the product by either forcing the board to overheat or causing problems in the operation of the circuit.

Using underfills for BGA’s and a conformal coating can help protect a circuit from these effects, but a maintenance schedule could still be needed to clean the boards.

An underfill is a liquid that fills in under a BGA or other leadless component to protect the joints and give extra mechanical rigidity.

Conformal coating are sprayed over a PCB to protect the board from moisture and other contaminants. There are many types to use and choosing the right one to protect against the contaminants it’s expected to encounter will need careful consideration.

Both of these processes should only be applied to a tested working board. If these are to be used on a cleaned board, the board must be really clean as any residue flux is likely to corrode any joints or copper over time (such is the nature of no clean fluxes). Once a conformal coating has been applied, there’s little that can be changed on a board, so it’s make or break. If all this works, your PCB will work in some harsh environments for a good time without trouble.

As is always discussed, how a PCB fabricator makes the PCB’s that you design can make or break a project. The same is true with reliability, knowing what to ask and what’s needed is most of the battle. If we’re hoping to create a circuit that is going to operate reliably in harsh conditions, what is it that we need to be looking for in our fabricator and asking them to do?

Finding the higher grade PCB materials for use in yours boards is all very well – but can your PCB fabricator get them and use them? Some of the cutting edge materials can be very hard to get hold of, or can have a long lead time attached to them. Similar products are often available from a fabricator if your needs are discussed, who knows, perhaps even something better.

With the materials decided, the circuit has to be manufactured in a way that will enhance it’s reliability. As always, it depends on the application, but if we’re going to consider harsh environments and temperature changes the considering the type of copper will make a difference.

Typically two types are available on PCB laminates, Electro Deposited (ED) and Rolled Annealed copper (RA). Both processes are self explanatory, but rolled annealed will typically be more robust as it’s a sheet of thinly rolled copper presses onto the laminate surface. These sheets will also be used when making inner layers on multilayer assemblies and bonded between layers of prepreg and core materials. It’s not generally known, but a tiny amount of lead is added to increase it’s durability, but not enough to break RoHS directives.

Electro deposited copper will be more fragile and fractures are more likely to be made when subjected to mechanical stresses such as thermal expansion and contraction.

The most likely part of a PCB to fail when subjected to thermal stress are the via’s. The z-axis will expand and could fracture via’s. The via wall thickness will thin in the centre, creating a weakness that can break in thermal cycling.

A fabricator will typically specify the thickness of via wall they’re able to manufacture. Specifying a minimum via wall thickness of 25um will reduce the risk of via fracture considerably. There is no way to eliminate this completely of course especially in cases of extreme temperature change.

The quality and accuracy of via drilling also has a distinct affect on via reliability. Drill bits need to be replaced a regular intervals to ensure the drill is sharp when every via is drilled. A blunt drill will create a rough via wall that could fail, especially when stressed.

Drill accuracy is how on target every via has been drilled. On larger via’s an pads it’s easy to see this, but on smaller.

The condition that needs to be avoided is called the keyhole effect (as shown above) where the drill hole is not on target and can create a potential weakness and failure effect where the track leaves the bad or via.

It needs noting that no via hole will ever be spot on – but it needs to be within the stated manufacturing tolerances of the fabricator. The IPC-A-600 standard will give guidance on the acceptability criteria of this an many other manufacturing defects that may arise from a fabricator. It will also give an idea of the acceptability criteria to specify when asking them to supply boards.

Designing electronics that works on the bench is one thing, but lets put these electronics in a harsh environments, which is dirty, where the temperature changes from hot to cold, perhaps rapidly. Maybe the equipment is going to be mounted in a helicopter and experience extreme vibration or perhaps in an environment that subjects the circuits to steam?

Every designer should have a kind of risk assessment in their heads of the factors every design has to face and one of these without a doubt is the expected lifetime of the product or equipment. For example, equipment that only needs to operate only a few hours for a race needs to be thought of differently to the reliability expected for a board operating on an aircraft without fail for 25 years.

The key to knowing how to design the electronics is to have a good knowledge of the environment the equipment is going to operate in. What are the temperatures that it’s expected to operate in? Vibration and mechanical shock are also issues as well as humidity and moisture. The possibilities can be daunting – but if a few simple things are done, the risks around many

Of these issues can be reduced or eliminated.

Temperature

The first thing to remember about temperature changes, is that everything moves. If warming up then all the materials expand, when cooling they contract. If the circuit undergoes rapid heating or cooling or both the board is going to expand and contact. While this isn’t a big issue in the x and y axis of the board, the z axis is different.

The values of thermal expansion of PCB materials is typically greater in the z axis, but this is widely disregarded as the distance is so short. But if a high degree of change takes place then via’s can fracture, often creating a very irritating failure that will be hard to find and harder to fix.

Reducing the total thickness of the board can help with this if cheaper materials need to be used. Using materials that are designed to operate in higher temperatures can be very effective as these have a reduced coefficient of thermal expansion. The extra cost of these enhanced FR4 materials is often minimal, it only get higher when considering much higher grade materials, the this cost is mostly lost in the processing of the PCB’s.

Another cost effective solution is to ask the fabricator to enhance the via wall thickness. This is covered more in the next article.

Lastly the right components need fitting to the board. If the electronics are going to be operating in an 80C environment, then 50C rated components aren’t going to be good enough. Make sure the components are rated for the task, or it’s likely that something weird will start happening at the operating extremes.

Mechanical Shock – Vibration

This is perhaps the hardest external factor to try and de-risk. Making sure that the pads are large enough, especially for components with a large mass is about all a designer can do. Only when carrying out drop and vibration testing can a designer or engineer get an idea of how well the electronics will hold together and survive in use.

Humidity & Moisture

However undesirable, humidity and moisture can be a big issue for electronics operating outside the home or office. Cars, planes, industrial equipment can all experience this. Where heat can create steam, any electronics subjected to steam are going to have a really tough time, especially if any other chemicals are in the steam as well. External factors like enclosing the electronics or conformally coating them can be used but add extra cost which can be undesirable in some cases. The best alternative is to monitor the moisture the board is subjected to, perhaps allowing extra features to shut down the circuit or warn the end user.

The question is how to do this cost effectively. Adding a conductive test coupon to act as a sensor to any kind of surface pollution is the first step. Adding it to either of the surface copper layers and leaving it exposed and coated in the same finish as the rest of the board will cost very little. Coupling this to a spare op-amp and digital I/O could mean that the only extra expense is the cost of designing it in and testing it.

Designing in reliability is more about thought and understanding than cost. More can be achieved using cheap and simple techniques before spending a lot more money – it just depends where the electronics going and how important it is that it keeps on going.